Development of a 0.5-m superconducting undulator prototype (SCU19) for field quality optimization

Superconducting undulators (SCUs) are expected to become key components of advanced photon sources, including synchrotron radiation sources and free-electron laser facilities. Compared to permanent magnet undulators, they can provide higher magnetic field strengths and a wider spectral range. However, achieving the stringent magnetic field quality remains a significant engineering challenge. This paper presents the development and testing of SCU19, a 0.5-m prototype designed to validate correction strategies for a future 3-m device targeting a peak field exceeding 1.07 T with a 19 mm period length. Unlike its predecessor, SCU15, SCU19 incorporates innovative structural designs, including the elimination of easily deformable long pole pieces and an enhanced epoxy impregnation process utilizing fiberglass powder to improve mechanical stability and excitation efficiency. Key magnetic field correction methods investigated include correction coils for managing field integrals and background interference, and a novel technique involving drilling holes in magnetic poles to insert adjustable soft iron columns for local field tuning. Experimental results from vertical measurements in liquid helium demonstrate the efficacy of these strategies. The implementation of end-correction coils significantly lowered the RMS phase error of SCU19, which was further minimized through subsequent adjustments with soft iron columns. These findings confirm that the proposed correction schemes and manufacturing improvements effectively address field quality limitations, laying a robust foundation for the engineering realization of high-performance SCUs comparable to permanent magnet devices.